The present invention generally relates to gradation (or density) control devices for thermal ink-transfer type printing apparatuses (hereinafter simply referred to as thermal printers), and more particularly to a gradation control device which controls the size of printing dots by an applying time of a constant current which is applied to heating elements of a thermal printing head in order to control the gradation level or printing density in a thermal printing apparatus.
Among therminal printers or hard-copy apparatuses such as wire-dot type and ink-jet type printers, thermal printers are being developed as one of the more promising type. For example, the thermal printer employs an ink film which is a polyester film having a thickness of 5 to 6 microns coated with a kind of ink which melts due to heat on one surface thereof. The ink film is placed onto a recording sheet with the ink side making contact with the recording sheet, and a thermal printing head makes contact with a rear side of the ink film. When a current flows through the thermal printing head so as to generate heat at the thermal printing head, the ink on the ink film melts at the position corresponding to the position of the thermal printing head, and the melted ink is transferred onto the recording sheet. The thermal printing head comprises a plurality of heating elements arranged in a row, and a current is successively applied to each of these heating elements.
The density which determines the gradation level of the printed characters, diagrams, pictures and the like, is determined by the area of each dot formed on the recording sheet due to the transfer of the melted ink onto the recording sheet. And, this area of the melted ink dot is determined according to the current applied to each of the heating elements. Generally, the heat value becomes larger as the magnitude of the current applied to the heating element becomes larger. As a result, the area of the melted ink dot becomes larger to increase the printing density, and the gradation level reaches near a saturated density. Accordingly, the magnitudes of the currents applied to the heating elements are conventionally controlled in order to control the gradation level of the printing. However, the currents applied to the heating elements are generally large currents in the order of 5 to 20 Amperes. Thus, it is difficult to control such large currents with a quick response speed, and there are disadvantages in that the size of the gradation control device becomes large and the gradation control device becomes expensive. Furthermore, it is impossible to increase the response speed when controlling such large currents, and there is a disadvantage in that the printing speed cannot be increased.
Accordingly, an improved tone (gradation) control device for a thermal printer was previously proposed in a U.S. Pat. No. 4,532,523 in which the assignee is the same as the assignee of the present application. This previously proposed tone control device controls the printing density by controlling the size of printing dots according to applying times of currents which are applied to the heating elements of the thermal printing head.
However, there is a problem in that the relationship between the printing density and the current applying time of the current which is applied to the heating element, that is, the relationship between the printing density and the heating time of the heating element, is not linear. In other words, the relationship between a maximum printing density obtained with a maximum heating time and a printing density obtained with a shorter heating time is non-linear, and an error is inevitably introduced between the printing density which is to be obtained and the printing density which is actually obtained.
Conventionally, as disclosed in a U.S. Pat. No. 4,399,749, there is a method of using as a reference clock pulse signal a signal which is obtained by frequency-dividing a pulse signal having a constant repetition frequency f1 with a dividing ratio which changes depending on a reference printing density datum, in order to eliminate the error in the printing density which is actually obtained. However, according to this system, it is necessary to transfer control data to a shift register (or a buffer register) at a maximum frequency of the reference clock pulse signal which changes depending on the reference printing density datum. For this reason, a maximum transfer speed of the control data becomes limited by devices which are used in the system. In this system, the frequency of the reference clock pulse signal is on the average considerably lower than a maximum allowable frequency of the devices which are used, and as a result, there is a problem in that it takes time to control the printing density. Consequently, it takes time to complete the printing operation.
In other words, since the system according to the U.S. Pat. No. 4,399,749 changes a frequency f2 of the reference clock pulse signal, a data transfer time for one printing density changes, where the data transfer time for one density is a sum of a time it takes to compare one printing density with hpicture element data of the dots corresponding to one printing line and a time it takes to transfer the compared results. Hence, a minimum heating time of the heating element is equal to the data transfer time for one printing density. For example, in the case where the maximum frequency of the reference clock pulse signal is 1 MHz, the number of heating elements is 256 and a buffer supplied with the compared results (control data) is constituted by a 256-bit shift register, the data transfer time for one printing density is 256 microseconds (=256/(1 MHz)). Accordingly, when a control range described by (maximum heating time)/(minimum heating time) is multiplied by ten, the data transfer time for a maximum printing density becomes 2.5 milliseconds (=256 microseconds.times.10) because the data transfer time for one printing density is equal to the minimum heating time. When an average of the data transfer time for one printing density is assumed to be 1.0 millisecond and the printing is carried out with 32 possible printing densities (that is, 32 gradations), it takes 32 milliseconds (=32.times.1.0 millisecond) to print the dots corresponding to one line. Therefore, this system has problems in that it takes a long time of approximately 32 milliseconds to print one line and the data transfer time for one printing density changes.